The invention relates to a solenoid valve, especially an outlet valve for infusion water, with a valve housing accommodating a valve chamber that communicates directly with a supply channel and by way of a valve seat with a drain channel, and with a magnetic system mounted on the valve housing with a magnetic coil accommodating a magnetic armature positioned in a positioning tube located opposite a head in the upper part of the magnet coil and connected to a magnet yoke, the armature also being connected at the end facing the valve seat to a valve plate by way of a valve shaft, and the valve chamber being sealed off from the interior of the positioning tube by an isolating diaphragm whose outer edge is connected to and seals off the valve housing and whose inner edge is connected to and seals off the valve plate.
Solenoid valves of this kind, of the so-called medium-isolated type, are used primarily in hot-beverage vending machines and are in themselves known. A valve of the aforesaid type is described for example in the prior DE-OS No. 3 613 481.
When the valves are used as outlet valves for infusion water, they are subjected to very high stress and must be overhauled regularly and even replaced after a relatively short service life.
It has been found that a number of factors are responsible for this high stress. When, for example, water with a high calcium content is used, calcium deposits form relatively rapidly in the valve housing and especially on the valve seat and valve plate. Initially this merely leads to a certain amount of leakage and not to complete failure of the valve. Since, however, the drops of water that occur as a result of leakage evaporate at the interface between the water-channeling part of the valve and the atmosphere, the salts that cause hardness in the water are deposited at that point, eventually resulting in complete failure of the valve. The process is accelerated by the high temperature of the medium.
In soft-water areas, calcination is of minor importance as a cause of failure. Instead, ferritic constituents form as a result of mechanical abrasion of the magnetic armature in the positioning tube and precipitate in the enclosed armature chamber. The result is a corrosion-prone powder that accumulates in the valve's magnetic system and corrodes there. This process is further accelerated by the fact that, despite the isolating diaphragm between the valve chamber and the interior of the positioning tube, moisture can eventually penetrate into the interior of the chamber. This is due to the fact that thin-walled diaphragms made of silicone material are used as isolating diaphragms. Water vapor, especially near the boiling point, can diffuse in small quantities through this material. A condensate accumulates on the side of the diaphragm that faces the interior of the guide tube. This condensate is calcium-free, but causes the ferritic material abraded from the magnetic armature to turn into iron oxide. As a result of this corrosion abrasion, depending on the frequency with which the unit is switched on, the magnetic armature can eventually clog up. Furthermore, as a result of the accumulation of the water of condensation at the point where the valve shaft is secured to the valve plate, which is generally integrated into the isolating diaphragm, changes in stroke can occur which result in inaccurate portion control.
In especially unfavorable cases, diffusion of water through the isolating diaphragm can even result in failure of the electric control system.
Finally, it has also been found that despite a high level of manufacturing precision, the magnetic armature tends to assume a preferred position in the positioning tube. As a consequence the valve plate is exposed to asymmetric contact pressure in the vicinity of the seal. The calcium deposits are thickest at the point of least contact pressure.